1. Technical Field
This invention relates to a tapered hollow shaft, and in particular to a tapered hollow shaft made of a fiber-reinforced composite material comprising reinforcing fibers having a specific tensile modulus.
2. Description of the Prior Art
A golf club receives a large shock, for instance, in the case of impacting a ball with a spot that is beside a sweet spot or in the case of hard hitting the ground, which is called "duff," in the swing. Particularly, the largest impact stress is produced at the junction between the golf shaft and the golf head, that is, in the vicinity of the hosel. Therefore, the use of conventional carbon fiber reinforced composite shafts, namely carbon shafts, results in that the breaking incident of the shaft is sometimes caused depending upon the condition of the shaft used by a user. In order to avoid such a breaking incident, many golf shaft manufacturers have come up with various ideas to improve the tip portion of the shaft in impact resistance.
Given as one instance of these ideas is a technique in which the tip portion of the shaft is reinforced using high strength PAN (polyacrylonitrile)-based carbon fibers which are said to have high impact resistance. This technique intends to improve the tip portion of the shaft in impact resistance by orienting the high strength PAN-based carbon fibers to be substantially parallel to the longitudinal direction of the shaft and disposing them at the tip portion of the shaft to form a reinforcing layer. The high strength PAN-based carbon fibers are carbon fibers which are widely used not only in the fields of golf shafts but also in various industrial and manufacturing fields as well as in sport and leisure products such as fishing rods, tennis rackets and badminton rackets at present.
The high strength PAN-based carbon fibers are characterized in that its tensile strength is outstandingly higher than that of conventional common carbon fibers. This is evidence to prove that the manufacturers of PAN-based carbon fibers believe that it is effective to improve the carbon fibers in tensile strength in order to better the carbon shaft in impact resistance. For instance, the strand tensile strength of Toray T700S (strand tensile modulus: 230 GPa) which is widely used as one of the high strength PAN-based carbon fibers is 4900 GPa. This is improved about 40% more than the strand tensile strength (3530 MPa) of Toray T300 (strand tensile modulus: 230 GPa) which has been widely used as a common grade. In this manner, the PAN-based carbon fibers have been remarkably improved in tensile strength. High performance grades that are further improved in the tensile strength are currently sold on the market.
Generally, carbon fibers are loosely classified into PAN (polyacrylonitrile)-based carbon fibers and pitch-based carbon fibers. It can be said in common to conventional PAN-based carbon fibers and pitch-based carbon fibers that the compressive qualities are lower than the tensile qualities.
Not only fiber reinforced composite materials, but also all materials are deformed by the application of stress to produce strain. To mention specifically, tensile stress causes tensile strain whereas compressive stress causes compressive strain. For instance, a shaft for use in a golf club is also deformed ark-like when impact-bending stress is produced in the shaft by impacting a ball or by hard hitting the ground. As a consequence, compressive strain is caused by the compressive stress on the side of the center of curvature of the arc and tensile strain is caused by the tensile stress on the side opposite to the center of curvature.
The inventors noted the fact that the conventional carbon shaft with the tip portion reinforced by high strength PAN-based carbon fibers produces flexural rupture by a rupture mode dominated by the compressive side. More concretely, the inventors paid attention to the fact that the shaft deformed arc-like by the application of impact stress produces compressive rupture from the side of the center of curvature of arc at which the compressive strain is produced, and they considered this is because the compressive rupture occurs preceding to tensile rupture since the compressive breaking strain the high strength PAN-based carbon fibers have is smaller than its tensile breaking strain.
The high strength PAN-based carbon fibers bear a maximum level of compressive breaking strain among carbon fibers currently used. For instance, the compressive breaking strain of Toray T700S (unidirectional composite plate) is 1.4% which is found to be higher than those of common grade PAN-based carbon fibers, e.g., Toray T300 (1.0%) and Toray M30S (0.9%) and those of high modulus PAN-based carbon fibers, e.g., Toray M40J (0.7%), Toray M46J (0.5%), Toray M50J (0.4%) and Toray M60J (0.25%). However, the tensile breaking strain of Toray T700S (unidirectional composite plate) is 1.7%, showing that the compressive breaking strain is lower than the tensile breaking strain. Therefore, even the shaft with the tip portion reinforced using high strength PAN-based carbon fibers produces flexural rupture dominated by the compressive side. Therefore the inventors considered that the excellent tensile strength of the high strength PAN-based carbon fibers is not reflected on an improvement in the impact resistance of the shaft and a fundamental improvement in the impact resistance has not been made. On the basis of the fact that any carbon fibers with a compressive breaking strain exceeding 1.4% has not been used usually, even if the tip portion of the shaft is reinforced by carbon fibers having the highest tensile strength, flexural rupture dominated by the compressive side is eventually caused since the compressive breaking strain is 1.4% or less. It is therefore considered that there is room for improvement in impact resistance of the conventional carbon shaft.
Moreover, the conventional carbon shaft with the tip portion reinforced by high strength PAN-based carbon fibers has another drawback that the reinforced section has only poor flexibility. The strand tensile modulus of the high strength PAN-based carbon fibers is in the range of 230 GPa to 240 GPa. These values are much higher than the tensile modulus (from 5 GPa to 160 GPa) of low modulus carbon fibers used in this invention. The conventional carbon shaft reinforced using high strength PAN-based carbon fibers to improve the tip portion of the shaft in impact resistance has the drawback that it is, as aforementioned, not only unimproved in the impact resistance of the shaft, but also increased in the flexural rigidity in the reinforced section, thereby impairing the flexibility.
The reinforcement of the tip portion of the shaft by using high strength PAN-based carbon fibers has the problem of impaired flexibility. In view of this problem, Fenton et al (U.S. Pat. No. 5,093,162) used glass fibers as reinforced fibers having a low tensile modulus to secure the flexibility of the tip portion of the shaft.
Because each strand tensile strength of glass fibers and aramid fibers is as high as 3100 MPa and 3600 MPa respectively, these low modulus reinforcing fibers have been considered to be able to impart flexibility to the reinforced portion and, at the same time, to have an effect on the improvement in the impact resistance of the tip portion of the shaft. However, the compressive breaking strain of a unidirectional composite plate comprising glass fibers is 1.3% and particularly the compressive breaking strain of aramid fibers which are said to have high impact resistance is only 0.36%. Even if the tip portion of the shaft is reinforced using glass fibers or aramid fibers, the shaft produces flexural rupture dominated by the compressive side the same as the cases where the tip portion is reinforced by using high strength PAN-based carbon fibers, giving rise to the problem that the high tensile strength of the glass fibers or aramid fibers is not sufficiently reflected on the improvement in the impact resistance of the shaft.
As outlined above, the use of reinforcing fibers having high tensile strength has been considered to be of importance to improve the carbon shafts in impact resistance. However, in the conventional carbon shaft, there was not used reinforcing fibers having a compressive breaking strain exceeding that (1.4% or less) of the PAN-based carbon fibers. Therefore, even if the tip portion of the shaft is reinforced by carbon fibers such as high strength PAN-based carbon fibers having the highest tensile strength, flexural rupture from the compressive side dominates resulting in insufficient improvement in the impact resistance. Moreover, when the tip portion of the shaft is reinforced using high strength PAN-based carbon fibers having high tensile modulus, thereby causing the flexural rigidity to be increased, bringing about the abuse that the flexibility is impaired.
Even in the cases where the tip portion of the shaft is reinforced by low modulus reinforcing fibers, such as glass fibers or aramid fibers to impart flexibility to the tip portion of the shaft, the flexibility can be imparted, but the impact resistance of the shaft is eventually unimproved.